All waste water treatment and effluent treatment plants will have screening systems upstream of core processes in order reduce the bigger particles being carried over to the main treatment. The influent sewage/effluent water is screened to remove all large objects like cans, rags, sticks, plastics, hair etc. carried in the stream. This is most commonly done with an automated mechanically raked bar screen in large plants. In smaller plants sometimes a manually cleaned screen may be used. Bar screens or mesh screens of varying sizes may be used to optimize solids removal.
If gross solids are not removed they become entrained in pipes and moving parts of the treatment plant and can cause substantial damage and bring inefficiency in the process.These particles are removed because they may damage pumps and other equipment. The solids are collected and later disposed in a landfill or incinerated.
The screening differ for the type of technology applied for the treatment of wastewater. The screening requirement for Conventional WWTP is different from the membrane based technologies, such as MBR (Membrane Bioreactor). In conventional systems the screening up to 10ø mm is considered sufficient. The FS (Flat Sheet) membranes require the screening to be done up to 3ø mm, while the HF (Hollow Fibre) membranes may require screening to be done to 1ø mm. The larger particles damage the membranes & hence properly designed pre-screening is essential for successful operation of the plant.
Generally when fine screening is required, a course screen is installed upstream to reduce load on the Fine screen equipment & subsequent frequent clogging issues.
The screening equipment is normally of closed construction to avoid odour issues as well the vector nuisance. Generally the screens are made by wedge wire, but in special cases they can be made of specific hole size to suit the requirement.
All the modern screening equipments are automatic so as to minimise human contact as well as process criticality. The preferred MOC (Material of Construction) is SS 316 to avoid corrosion/erosion problems in long run.
The velocity of the influent passing through the bar screen should be such that it does not produce excessive loss of head or clogging of the screen. Acceptable values range between 0.5 – 1.5 m/sec. The upper limit of 1.5 m/s being applicable for the peak flow. The velocity figures are applicable for clear area of the screen. Normally 25 -30 % blockage is expected for Effluents with heavy solids load. For raw water the blockage would be lesser.
Generally the screening systems are cleaned either timer based or based on the pressure differential across or combination of both. The screening equipment is provided with electronic overload protection as well as torque limiter to protect damage in case of overload.
Various types of screening equipments have been developed based on the requirement; few of them have been described below.
BAR SCREENS
The bar screen is typically designed to prevent clogging of the fine screens. The trash collected on the grid is automatically removed at intervals by the cleaning rake. The cleaning phase of the bar screen is switched on by pressure sensors or timers. Normally two types are in use
- Inclined plate separator
- Curved separator
TRAVELLING BASKET FILTERS
If water is taken directly from a river, lake or sea by gravity flow, travelling basket filters are recommended for the treatment, especially if there are large fluctuations in the influent load.
The filter screen has a mesh size from 0.2mm to 3mm, depending on the purpose. The screen is generally made of stainless steel. The travelling basket filters can designed up to a capacity of 50,000m³/h.
DRUM BASKET FILTERS
Drum basket filters are used when a high degree of mechanical purification is required.
These filters have proven their reliability in a large number of installations worldwide. They are able to separate particles down to 50µm, and the mesh of the filters is selected according to the requirement.
